Method and apparatus for transmitter power control in a cellular radio system

Abstract: The object of the invention is a cellular radiotelephone system and a radiotelephone for it. Transmission (TX) and reception (RX) is in the system effected in different time slots, and the same centre frequency and bandwidth are used for transmission and reception. The radiotelephone measures (PM) the level of the signal (IRX) it receives from the base station and, in accordance with said received signal level, controls the power of the signal (ITX) to be transmitted in the transmit time slot immediately following said reception time slot.

Description: The present invention relates to transmit power control apparatus and method for a radiotelephone operable in a radiotelephone system utilising substantially the same centre frequency and bandwidth for both signal transmission and reception. In a cellular radio system based on time division multiple access (TDMA) or frequency division multiple access (FDMA), the control of transmit power is not necessary regarding the system operation. The first systems thus used the same transmit power all the time. In present cellular radiotelephone systems (e.g. NMT) control of the cellular radiotelephone power is used basically in order to save battery capacity. Power control is further used in small cells in order to reduce the noise level towards adjacent cells. However, usually there are only a few (about 5 to 10) usable power levels. In a system based on code division multiple access (CDMA), in which the spectrum is spread with the DS technology (Direct Sequence), the power received by the base station from different cellular radiotelephones should be approximately equal, so that the system can accommodate as many users as possible. In the CDMA system the primary object of the power control is thus to maximize the capacity of the system. Then the power level must be controlled very accurately, and the transmit level must be minimized. The transmit power can be controlled according to two different principles, either as open loop control or as closed loop control. In the open loop control the telephone measures the signal power received from the base station and then controls its transmit power accordingly. Generally the method does not, however, provide a good measurement accuracy, and thus it is neither possible to fully minimize the transmit power, so that it would not be inadvertently controlled too low due to the measurement inaccuracy. The reason for this is that the transmitted and received signals are differently attenuated, because they are situated at different frequencies. The difference can be 10 to 40 dB, depending on the radio signal's bandwidth and the propagation conditions. Thus the transmit power must be dimensioned to be sufficiently high, so that a reliable transmit link can be created. Open loop control is generally used in the present FDD/FDMA and FDD/TDMA systems (FDD, frequency division duplex), in which the transmit and reception is effected at different frequencies. The open loop control method is simple, but its disadvantage is the measurement inaccuracy of the method. US-4,811,421 describes means for estimating signal power to be received. A signal power to be received is estimated as a function of already received and measured signal. At each predetermined instant, a signal power, which will be received by the station at a following predetermined instant, is estimated, to thereby derive an estimated power. A signal power that the station transmits between the successive instant is controlled in proportion to the estimated power. In addition to the estimate, the system utilises different frequencies in signal transmission and reception. US-5,093,840 describes an apparatus for adaptive-power control of a spread-spectrum transmitter of a mobile station operating in a cellular-communications network using spread-spectrum modulation. A base station transmits a generic spread-spectrum signal and an APC-data signal. A mobile station has an acquisition circuit for acquiring and decoding the generic spread-spectrum signal, a detector for detecting a received power level of the generic spread-spectrum signal, a decoder for decoding the APC-data signal as a threshold, a differential amplifier for generating a comparison signal by comparing the received power level to the threshold, a transmitter for transmitting a transmitter spread-spectrum signal, an antenna, and a variable-gain device responsive to the comparison signal indicating an increase or decrease for adjusting a transmitter-power level of the transmitter spread-spectrum signal. A publication in IEEE International Conference on Communications 1986, "Direct-Sequence Spread Spectrum with DPSK Modulation and Diversity for Indoor Wireless Communications," (Bhaskar Ramamurthi and Mohsen Kavehrad), Toronto, Canada, June 22-25, 1986 describes Direct Sequence Spread-Spectrum Multiple Access (DS-SSMA) system using differential phase shift keying (DPSK) modulation for wireless communication in an indoor environment. The central station continuously transmits a one PN-sequence common to all the receivers. At the user location, a sensor constantly monitors the average power level in the received tone or modulated tone. Since the channel is reciprocal, the transmitter at the user location adjusts the transmitted power level proportional to the inverse of the average power level received by the sensor. A IEEE publication "Spread spectrum S-CDMA for personal communication services," Military communications conference, 1992, MILCOM '92 Conference Record, Communications - Fusing Commend, Control and Intelligence., IEEE San Diego, CA, USA 11-14 Oct, 1992, NY, USA, IEEE, US, pp. 269-273 describes the capacity advantage of spread spectrum CDMA radios over conventional FDMA and TDMA for personal communication services, The radios use DS/OPSK spread spectrum signals and TDD to achieve full duplex operation. In the closed loop control system the signal power received from the cellular radiotelephone is measured at the base station and the control information required for the control of the transmit power is transmitted to the cellular radiotelephone. The disadvantage of the closed loop control method is that the method is more complicated than the open loop control, because in the closed loop system additional information must be transmitted separately and the transmit of control information in the cellular radio system requires a transmit rate of about 1 kbit/s so that the control system can operate correctly. A control system of this type is proposed to be used in FDD/CDMA systems. A first aspect of the present invention provides transmit power control apparatus for a radiotelephone operable in a time division duplex/code division multiple access (TDD/CDMA) radiotelephone system utilising substantially the same centre frequency and bandwidth for both signal transmission and reception, comprising signal measuring means (PM) for measuring a received signal level, wherein the level of the received signal (IRX) is measured after signal dispreading, and power control means for controlling the level of a corresponding subsequent transmitted signal (ITX) in accordance with the received signal level, and a second aspect of the present invention provides a method of controlling transmit power for a radiotelephone operable in a time division duplex/code division multiple access (TDD/CDMA) radiotelephone system utilising substantially the same centre frequency and bandwidth for both transmission and reception, comprising measuring a received signal, wherein the level of the received signal (IRX) is measured after signal despreading, and controlling the level of a corresponding subsequently transmitted signal in accordance with the received signal level, further comprising absolute power measurement means for receiving information indicative of the level of the absolute power transmitted by a base station, and wherein the level of the corresponding subsequent transmitted signal (ITX) is controllable in accordance with the level of the absolute power transmitted by a base station and the received signal level. These have the advantage in that accurate power measurement and control can be achieved by simple circuitry and arrangements. In single frequency TDD system, such as the DECT system, the signal is attenuated in the same way in both transmission directions. In an apparatus and method in accordance with the invention the transmit power is set in accordance with the signal level received in the latest time slot, because the control of an open loop is very accurate when transmission and reception is effected at the same frequency in consecutive time slots. Then the information about the changes in the signal caused by the changes is the most recent, whereby the transmit power can be minimized very accurately. Usually the transmit power is controlled about 20 dB higher than the required level, so that the signal certainly is sufficiently strong. It is possible to set the transmit power accurately to the minimum, or to control the signal to be only slightly higher than the required level because, according to the invention, the measurement and the power control can be made very accurately. Thanks to the invention it is possible to realize a radiotelephone system, which has the accuracy of the closed loop control method and the simple embodiment of the open loop control. The radiotelephone can be realized so that it contains a preset reference value with which the received signal power level is compared, and the transmitted signal is controlled according to this comparison. Alternatively it could be realized so that the transmitter identifies its absolute transmit power at the transmit time and e.g. only when the transmit power changes, so that no high data communication rate is required. The received signal power level is preferably measured during the whole reception time slot, and from this the power can be calculated, for instance by averaging. Thus in the TDD system the signal is equally attenuated in both transmit directions, because the same frequency band is used in both directions (i.e. the same centre frequency and the same bandwidth). As the channel changes slightly as a function of time, the duplex time (the period of the transmit/reception time slot) must be sufficiently short, so that the change does not significantly influence the level of the signal, but accurate information about the received signal power level is obtained. The magnitude of the channel change depends e.g. by the fact whether the radiotelephone is moving or not (e.g. a car telephone). The invention is now described with reference to the enclosed drawings, in which: figure 1 shows the frame structure of the TDD/TDMA system, and an embodiment of the method according to the invention in the TDD/TDMA system; figure 2 shows the frame structure of the TDD/CDMA system, and an embodiment of the method according to the invention in the TDD/CDMA system; and figure 3 shows a block diagram of a circuit for the power control of a radiotelephone in accordance with the invention. Figure 1 shows a TDD/TDMA system, in which for example twelve radiotelephones communicate with the same base station. The first cellular radiotelephone measures the received signal level in the latest receive time slot 0, and it uses this information to control the transmit signal power immediately in the telephone's next transmit time slot 12. The second cellular radiotelephone correspondingly measures the received signal level in the latest receive time slot 1, and it uses this information to control the transmit signal power immediately in the telephone's next transmit time slot 13, and so on, whereby the last or twelfth cellular radiotelephone shown in figure I measures the received signal level in the latest receive time slot 11, and it uses this information to control the transmit signal power immediately in the telephone's next transmit time slot 23. The time slots 0 to 23 in figure 1 represent the fact that the base station pro cesses the transmission and reception of each of the twelve radiotelephones in different time slots. Figure 2 shows a TDD/CDMA system, in which the transmission and reception is effected in different time slots. The cellular radiotelephones measure the level of the re ceived signal R in the reception time slot and use this information to control the power of the transmit signal T in the transmit time slot. Figure 2 shows how in the TDD/CDMA system the base station processes simultaneously the transmission and reception for all twelve radiotelephones. However, the transmission and reception are effected in different time slots to each other. Figure 3 shows a diagram of a block circuit required for power control in accordance with the invention. The level of the information signal IRX arriving at the receiver RX is measured by the power measurement circuit PM. The second branch from the receiver RX represents the normal processing of the received information IRX in the radiotelephone, of which it is not necessary to present a more detailed block diagram in this connection as it is well known to a person skilled in the art. The level of the signal can be measured by measuring either its amplitude, power or the like by a convenient known method. The level of the received signal is preferably measured during the whole reception time slot, and from this the power can be calculated, e.g. by averaging. The measurement period must be selected to be sufficiently long so that the effect of any noise will be small due to the effects of averaging. However, the measurement period must also be short enough so that the data communication channel does not have time to change substantially during the measurement. The measurement result is compared with a reference value SRE F, and the power of the transmitter TX is adjusted accordingly. The reference value SRE F can be any preset reference value with which the level of the received signal IRX is compared in the comparison means COMP, and the level of the transmitted signal ITX is adjusted under the control of signal SREG according to this comparison. Alternatively the reference value SRE F can be arranged to the comparison means so that the transmitter detects the absolute transmit power transmitted at the base station at the transmit time, and e.g. only when the transmit power changes, so that no high data communication rate is required. A radiotelephone system in accordance with the invention provides the accuracy of the closed loop control method, even though no control information is transmitted from the base station to the cellular radiotelephone. With the method of the invention the transmit power of the cellular radiotelephone can be minimized and thus the power consumption is minimized. The interference of the cellular system also decreases because of lower transmit powers. There will also be a smaller distance for the repeated allocation of the same channel (i.e., the frequency band in FDMA, the code in CDMA and the time slot in TDMA), whereby the system can contain more users for each frequency unit and area unit. The number of users can be increased with dynamic channel allocation (DCA) due to the reduced interference obtained with the power control according to the invention. It is also possible to obviate the effects of fading by adjusting a suitable power level. The method of the invention can be applied in a system using time division, in which also the transmit and reception is effected at the same frequency, e.g. in the TDD/CDMA or TDD/TDMA multiple access method. The power of a signal received in the CDMA system or in another spread spectrum system is measured when the signal has been despread (the signal is correlated by the spreading code). The invention is very advantageous in the CDMA system, where it is important that the signal received by the base station is approximately equally strong from all mobile stations. The signal power can be measured either from the R F signal, from the IF signal or from the demodulated signal (in the TDMA system). In the DECT cellular radio system both the mobile station and the base station (transmitter and receiver) control their power levels independently. In the DECT system the speech packet is coded at the transmission (CRC, Cyclic Redundancy Check) in order to detect transmit errors at the reception. CRC is an error detection system, in which parity bits are generated by encoding, and the errors created during the transmission are detected by a decoding algorithm. When the receiver detects an erroneous speech packet, it requests repeated transmission, whereby the transmitter receives immediately in the next time slot information about a successful reception. Thus in the power control of the transmit signal it is possible to take into account the proportional ratio of successful transmissions in order to increase the success percentage, and at least in order to remove errors caused by the (too low) power level by using the power control method of invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention as defined by the appended claims.

Claims: Transmit power control apparatus for a radiotelephone operable in a time division duplex/code division multiple access (TDD/CDMA) radiotelephone system utilising substantially the same centre frequency and bandwidth for both signal transmission and reception, comprising signal measuring means (PM) for measuring a received signal level, wherein the level of the received signal (IRX) is measured after signal despreading, and power control means for controlling the level of a corresponding subsequent transmitted signal (ITX) in accordance with the received signal level, characterized in that , the transmit power control apparatus further comprises absolute power measurement means for receiving information indicative of the level of the absolute power transmitted by a base station, so that the level of the corresponding subsequent transmitted signal (ITX) is controllable in accordance with the level of the absolute power transmitted by a base station and the received signal level. Apparatus according to claim 1, wherein the level of the received signal (IRX) is compared with a predetermined reference level (SREF), and the level of the corresponding subsequent transmitted signal is controllable in accordance with a result of the comparison (COMP). Apparatus according to any previous claim, wherein the transmit power level is controllable (SREG) to a level as low as possible at which the signal (ITX)propagates error-free to the base station. Apparatus according to claim 1, wherein the level of the received signal (IRX) is measured from the radio-frequency signal. Apparatus according to claim 1, operable for a radiotelephone system having transmit and receive time slots that have substantially the same length in time as a measured period during which the measurement of the received signal (IRX) takes place. A cellular radiotelephone comprising apparatus as claimed in any preceding claim. A method of controlling transmit power for a radiotelephone operable in a time division duplex/code division multiple access (TDD/CDMA) radiotelephone system utilising substantially the same centre frequency and bandwidth for both transmission and reception, comprising measuring a received signal (IRX), wherein a level of the received signal (IRX) is measured after signal despreading, and controlling the level of a corresponding subsequently transmitted signal (ITX) in accordance with the received signal level, characterized in that , the method further comprises receiving information indicative of the level the absolute power transmitted by a base station and controlling the level of the subsequently transmitted signal in accordance with the absolute power and the received signal level. A method according to claim 7, wherein the level of the received signal (IRX) is compared with a predetermined reference level (SREF), and the level of the corresponding subsequent transmitted signal is controllable in accordance with a result of the comparison (COMP). A method according to any of claims 7 to 8, wherein the transmit power level is controllable (SREG) to a level as low as possible at which the signal (ITX) propagates error-free to the base station. A method according to claim 7, wherein the level of the received signal (IRX) is measured from the radio-frequency signal. A method according to claim 7, operable for a radiotelephone system having transmit and receive time slots that have substantially the same length in time as a measurement period during which the measurement of the received signal (IRX) takes place.

Inventors: JUNTTI JUHANI, JUNTTI, JUHANI
Applicants: NOKIA CORP, NOKIA CORPORATION
Application year: 1994
Main classification: H04B7
IPC codes: H04L5, H04B7, H04J13